In the manufacturing process of a nonvolatile semiconductor memory device, the chemical mechanical polishing (CMP) method may be used in order to form an element isolation region. There are element isolation regions with a wide width and a narrow width. If an element isolation region with a particularly wide width is processed by CMP, an erosion effect in which the surface of the element isolation region is recessed may occur. The erosion effect may influence other layers formed at a periphery of the element isolation region.